System and method of x-ray flux management control

ABSTRACT

A system and method of diagnostic imaging is provided that includes determining a position of a subject in a scanning bay and tailoring x-ray attenuation such that the specific position of the subject is taken into consideration. The present invention automatically selects a proper attenuation filter configuration, corrects patient centering, and corrects noise prediction errors, thereby increasing dose efficiency and tube output.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention claims the benefit of U.S. Ser. No. 60/514,711filed Oct. 27, 2003.

BACKGROUND OF THE INVENTION

The present invention relates generally to diagnostic imaging and, moreparticularly, to a method and apparatus to optimize dose efficiency bydynamically filtering radiation emitted toward the subject duringradiographic imaging in a manner tailored to the position and/or shapeof the subject to be imaged.

Typically, in computed tomography (CT) imaging systems, an x-ray sourceemits a fan-shaped beam toward a subject or object, such as a patient ora piece of luggage. Hereinafter, the terms “subject” and “object” shallinclude anything capable of being imaged. The beam, after beingattenuated by the subject, impinges upon an array of radiationdetectors. The intensity of the attenuated beam radiation received atthe detector array is typically dependent upon the attenuation of thex-ray beam by the subject. Each detector element of the detector arrayproduces a separate electrical signal indicative of the attenuated beamreceived by each detector element. The electrical signals aretransmitted to a data processing system for analysis and subsequentimage reconstruction.

There is an increasing desire to reduce radiation expose to a patientduring radiographic data acquisition. It is generally well known thatsignificant radiation or “dose” reduction may be achieved by using anattenuation filter to shape the intensity profile of an x-ray beam.Surface dose reductions may be as much as 50% using an attenuationfilter. It is also generally known that radiation exposure for dataacquisition from different anatomical regions of a patient may beoptimized by using specifically shaped attenuation filters tailored tothe anatomical region-of-interest (ROI). For example, scanning of thehead or a small region of a patient may be optimized using a filtershape that is significantly different than a filter used during dataacquisition from the heart. Therefore, it is desirable to have animaging system with a large number of attenuation filter shapesavailable to best fit each patient and/or various anatomical ROIs.However, fashioning an imaging system with a sufficient number ofattenuation filters to accommodate the numerous patient sizes and shapesthat may be encountered can be impractical given the variances in apossible population. Additionally, manufacturing an imaging system witha multitude of attenuation filters would increase the overallmanufacturing cost of the imaging system.

Further, for optimum dose efficiency, i.e. best image quality at thelowest possible dose, the attenuation profile created by the attenuationfilter should be particular to the patient. That is, it is desirable andpreferred that when selecting a pre-patient or attenuation filter thatit be adjusted according to the particulars of the patient, such as thepatient's size, shape, and relative position in the bore of the scanner,be taken into account. By taking these and other particulars intoconsideration, radiation exposure can be optimized for the patient andthe scan session.

Known CT scanners use both an attenuation filter and dynamic currentmodulation to shape the intensity of the x-ray beam incident to thepatient. To reduce radiation exposure, the attenuation filer istypically configured to minimize x-ray exposure to edges of the patientwhere path lengths are shorter and noise in the projection data has aless degrading impact on overall image quality. Accordingly, one suchimplementation of the attenuation filter is the bowtie filter, which, asa function of form, increases attenuation of x-ray intensity incidentupon of the peripheral of the imaging subject. However, improper patientcentering and/or bowtie filter selection can significantly degrade imagequality and dose efficiency because x-ray attenuation is misapplied tothe particulars of the subject. The bowtie filter is aligned with apoint of maximum radiation dose or isocenter. The bowtie filterminimizes attenuation of x-ray intensity to isocenter and attenuatesradiation significantly with radial distance beyond the center region ofthe bowtie, because, ideally, the isocenter corresponds to an imagingcenter of the subject. However, this is not always the case, e.g. whenthe subject is mis-centered in the scanner.

FIG. 1 illustrates a bowtie filter ideally matched to a patient.Specifically, bowtie filter 10 is aligned within an imaging beam 12 suchthat an x-ray profile 14 is generated by the incidence of the imagingbeam 12 upon the patient 16. However, if the patient 16 is not centeredwith respect to the bowtie center and the corresponding isocenter,significant image degradation can occur. The degradation is dependentupon a multitude of factors, such as the size of the central region ofthe bowtie filter, size and shape of the patient, and the amount anddirection of patient mis-centering. FIG. 2 illustrates one such exampleof a bowtie filter opening that is improperly matched to the patient.That is, the bowtie filter 10 is aligned within the imaging beam 12 suchthat an improper x-ray profile 18 is generated by the incidence of theimaging beam 12 upon the patient 16. Specifically, photon incidence orflux at the edges of the patent may increase image noise to a level thatmay be prohibitively high for diagnostically valuable images.

Recent improvements in imaging devices include a continuously adjustablebowtie filter having a pair of filtering elements to compensate forfactors that may lead to non-ideal imaging. Such a filter is describedin U.S. Ser. No. 10/605,789, the disclosure of which is incorporatedherein and is assigned to GE Medical Systems Global Technology Co., LLC,which is also the Assignee of this application. Each filter element hasa long low attenuating tail section that varies in attenuation poweracross its length such that as the elements are moved relative to oneanother, the attenuation of the beam is controlled. Each filter elementis dynamically positioned with a dedicated motor assembly. The filterelements may be positioned in the x-ray beam so as to shape the profileof the x-ray beam to match a desired ROI or anatomicalpoint-of-interest.

The filter portions are positionable and adjustable using precisionpositioners to control the radiation pattern for the patient or theanatomy currently being imaged. However, image degradation may occur ifthe bowtie opening created is too small for a large patient since usefulx-ray needed for imaging is attenuated by the bowtie thereby causinghigh image noise. As a result, the operator must manually determine theappropriate beam width and position according to size, shape, andpositioning of the subject within the scanner bore.

A properly sized bowtie configuration, however, does not ensureacceptable image quality. If the subject is mis-centered, imagedegradation may still persist. This degradation is typically a result oftwo factors. First, if subject mis-centering is caused by mis-elevationof the subject with respect to the bowtie filter then the calculation oftube current will result in an underestimate of the subject size.Referring to FIG. 3, a patient 16 is shown mis-centered in an x-ray beam12. Specifically, the patient 16 is positioned at an improper centeringelevation 20 by a centering error 22 below a proper centering elevationor y-position 24. As a result, a portion of the imaging beam 26 is notincident upon the patient 16 and a projection area 28 is understated byan error margin 30 because the patient 16 intercepts fewer rays in theimaging beam 12. As such, when determining tube current with the imagingtube at top-dead-center, as is convention, a lower tube current thanactually required for proper imaging will be determined. As a result ofthe lower tube current, excessive image noise will be present relativeto the user's selection. For example, a calculated milliamp (mA) that is30% lower than actually required for proper imaging occurs for a typical30 cm×20 cm body mis-centered in elevation by three cm. In such a case,noise introduction is increased by approximately 15% from a properlycentered, properly imaged, patient.

Secondly, patient mis-centering with respect to elevation may alsoposition the thickest part of the patient such that x-rays for lateralprojections pass through the thickest part of the bowtie which resultsin over-attenuation of the imaging beam. Referring to FIG. 4, thepatient 16 is shown mis-centered within the imaging beam 12 by acentering error 22 below the proper centering elevation 24. As a result,the imaging beam 12 passes through the thickest parts of the bowtiefilter 10 and patient 16, as exemplified by projection route 32. Suchmis-centering can result in an additional image noise increase by asmuch as 70%. These errors can cause images of such high noise that thediagnostic value is compromised. Moreover, since traditional CT imagingmethods rely on operator input to perfect patient centering, includingelevation, elevational patient mis-centering can be common. Furthermore,traditional edge detection methods rely on identifying the center of thepatient indirectly by detecting the edges of the patient, which can beparticularly susceptible to error.

Additionally, recent advancements in detector technology has increasedthe desire to control x-ray flux management to within very accurateconstraints. For example, photon counting (PC) and energy discriminating(ED) detector CT systems have the potential to greatly increase themedical benefits of CT by differentiating materials such as a contrastagent in the blood and calcifications that may otherwise beindistinguishable in traditional CT systems. Additionally, PC and ED CTsystems produce-less image; noise for the same dose than photon energyintegrating detectors and hence can be more dose efficient thanconventional CT systems. However, while PC and ED CT systems have thepotential to realize numerous advantages over traditional CT detectors,the systems may be impractical for some scan protocols.

Therefore, it would be desirable to design an apparatus and method toautomatically control flux by dynamically filtering radiation emittedtoward the subject during radiographic imaging in a manner tailored tothe position and/or shape of the subject to be imaged so as to optimizeradiation exposure during data acquisition. It would be furtherdesirable to have a system that tailors the radiation emitted toward thesubject during data acquisition based on a scout scan of the subject.Furthermore, it would be advantageous to have a system and method ofcontrolling x-ray flux management to avoid photon pileup. Additionally,it would be desirable to have a system and method of dynamicallyadjusting radiation filtering to follow a user defined-region-ofinterest. It would also be desirable to have an apparatus toautomatically collect patient centering and surface elevationinformation include a direct method of detecting patient centering.Furthermore, it would be desirable to have a method of accuratelydetermining patient mis-centering within an imaging volume and adjustingthe patient position to compensate for the determined mis-centering.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is a directed method and apparatus to optimizeradiation exposure that overcomes the aforementioned drawbacks. Thepresent invention includes a method and apparatus to automaticallycontrol x-ray flux by dynamically filtering radiation emitted toward thesubject during imaging in a manner tailored to the position and/or shapeof the subject to be imaged. A method of determining the shape andrelative position of the subject prior to CT data acquisition is alsodisclosed.

In accordance with one aspect of the invention, a method of diagnosticimaging is disclosed including determining a position of a subject in ascanning bay relative to a reference position, automatically adjustingan attenuation characteristic of an attenuation filter based on thedetermined position of the subject, and imaging the subject.

In accordance with another aspect of the invention, a computer readablestorage medium is disclosed having stored thereon a computer programrepresenting a set of instructions. When the instructions are executedby at least one processor, the at least one processor is caused toreceive feedback regarding mis-centering of a subject to be scanned,determine a value of mis-centering of the subject to be scanned, andadjust at least one of an attenuation filter configuration and a subjectposition based on the value of mis-centering. The processor is thencaused to acquire radiographic diagnostic data from the subject.

In accordance with another aspect of the invention, a tomographic systemis disclosed. The tomographic system includes a rotatable gantry havinga bore centrally disposed therein, a table movable within the bore andconfigured to position a subject for tomographic data acquisition withinthe bore, and a high frequency electromagnetic energy projection sourcepositioned within the rotatable gantry and configured to project highfrequency electromagnetic energy toward the subject. A detector array isdisposed within the rotatable gantry and configured to detect highfrequency electromagnetic energy projected by the projection source andimpinged by the subject and an attenuation filter positioned between thehigh frequency electromagnetic energy projection source and the subject.A computer is programmed to adjust at least one of an attenuationcharacteristic of the attenuation filter and a table position based on aspecific position of the subject in the bore.

Various other features, objects and advantages of the present inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one preferred embodiment presently contemplatedfor carrying out the invention.

In the drawings:

FIG. 1 is a schematic view of a properly aligned attenuation filterassembly and a resulting x-ray profile.

FIG. 2 is a schematic view of an improperly aligned attenuation filterassembly and a resulting x-ray profile.

FIG. 3 is a schematic view of an improperly centered patient within animaging beam and a resulting projection area.

FIG. 4 is a schematic view of an improperly centered patient within animaging beam.

FIG. 5 is a pictorial view of a CT imaging system.

FIG. 6 is a block schematic diagram of the system illustrated in FIG. 5.

FIG. 7 is a plan view of a representative x-ray system.

FIG. 8 is a sectional view of a portion of the x-ray system shown inFIG. 5.

FIG. 9 is a flow chart showing a process in accordance with the presentinvention that may be implemented with systems of FIGS. 5–8.

FIG. 10 is a schematic view of an attenuation filter assembly with animproperly centered patient and the resulting flux profile.

FIG. 11 is a flow chart showing a process for selecting an attenuationfilter configuration and tube current modulation scheme utilizing AP andlateral scout scans.

FIG. 12 is a graph of a set of optimum bowtie opening values derivedfrom patient size.

FIG. 13 is a graph of a quality factor versus attenuation filter openingfor a plurality of patient sizes.

FIG. 14 is an illustration of a lateral scout scan showing an improperlycentered patient.

FIG. 15 is an illustration of a user-defined ROI by placement ofreference markers on an interface in accordance with one aspect of thepresent invention.

FIG. 16 is a flow chart showing a process for selecting an attenuationfilter configuration and tube current modulation scheme utilizing alateral scout scan.

FIG. 17 is a flow chart showing a process for selecting an attenuationfilter configuration and tube current modulation scheme utilizing an APscout scan.

FIG. 18 is an illustration of surface elevation derivation with knownpatient height data in accordance with the present invention.

FIG. 19 is an illustration of surface elevation derivation with unknownpatient height data in accordance with the present invention.

FIG. 20 is a schematic view of a sensor assembly incorporated into animaging scanner for derivation of patient surface elevation.

FIG. 21 is a pictorial view of a CT system for use with a non-invasivepackage inspection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to a method and system thatautomatically determines the patient's size, shape, and centering withinan imaging volume and dynamically controls x-ray flux accordingly.Preferably, one or two scout scans together with a plurality of sensorsintegrally formed with the CT scanner provide patient particulars. Thepresent invention uses the information to provide centering informationto the user, allow user input, automatically re-center the patientelevation, correct projection area measurements for dynamic tube currentcontrol and select the correct bowtie filter for the optimum doseefficiency.

The operating environment of the present invention is described withrespect to a four-slice computed tomography (CT) system. However, itwill be appreciated by those skilled in the art that the presentinvention is equally applicable for use with single-slice or othermulti-slice configurations. Moreover, the present invention will bedescribed with respect to the detection and conversion of x-rays.However, one skilled in the art will further appreciate that the presentinvention is equally applicable for the detection and conversion ofother high frequency electromagnetic energy. The present invention willbe described with respect to “third generation” CT systems but isequally applicable with a wide variety of CT systems. That is, it iscontemplated that the present invention may be utilized with energyintegrating, photon counting (PC), and/or photon energy discriminating(ED) CT detector systems.

Specifically, it should be recognized that the present inventionprovides a technique that controls and effectively limits detectorsaturation. The technique is adaptable such that it may be tailored tospecific the requirements and constraints of a particular detector typeand/or detector arrangement. For example, energy integrating detectors,which integrate the amount of x-ray flux recorded during an exposuretime, have an inherent flux level tolerance that is relatively high. Onthe other hand, direct conversion detectors such as photon countingdetectors, which actually count each photon as it passes, have a verydifferent, typically lower, flux level tolerance. The present inventionprovides a dynamically adaptable technique whereby specific flux leveltolerances may be observed to avoid detector saturation.

Referring to FIGS. 5 and 6, a computed tomography (CT) imaging system100 is shown as including a gantry 102 representative of a “thirdgeneration” CT scanner. Gantry 102 has an x-ray source 104 that projectsa beam of x-rays 106 through a filter assembly 105 toward a detectorarray 108 on the opposite side of the gantry 102. Detector array 108 isformed by a plurality of detectors 110 which together sense theprojected x-rays that pass through a medical patient 112. Each detector110 produces an electrical signal that represents the intensity of animpinging x-ray beam and hence the attenuated beam as it passes throughthe patient 112. Moreover, the detectors may be photon energyintegrating detectors, photon counting, and photon energy discriminatingdetectors. During a scan to acquire x-ray projection data, gantry 102and the components mounted thereon rotate about a center of rotation114.

Rotation of gantry 102 and the operation of x-ray source 104 aregoverned by a control mechanism 116 of CT system 100. Control mechanism116 includes an x-ray controller 118 that provides power and timingsignals to an x-ray source 104, a gantry motor controller 120 thatcontrols the rotational speed and position of gantry 102, and filterassembly controller 123 that controls filter assembly 105. A dataacquisition system (DAS) 122 in control mechanism 116 samples analogdata from detectors 110 and converts the data to digital signals forsubsequent processing. An image reconstructor 124 receives sampled anddigitized x-ray data from DAS 122 and performs high speedreconstruction. The reconstructed image is applied as an input to acomputer 126 which stores the image in a mass storage device 128.

Computer 126 also receives commands and scanning parameters from anoperator via console 130 that has a user interface device. An associatedcathode ray tube display 132 allows the operator to observe thereconstructed image and other data from computer 126. The operatorsupplied commands and parameters are used by computer 126 to providecontrol signals and information to DAS 122, x-ray controller 118 andgantry motor controller 120. In addition, computer 126 operates a tablemotor/table centering controller 134 which controls a motorized table136 to position patient 112 and gantry 102. Particularly, tablemotor/table centering controller 134 adjusts table 136 to move portionsof patient 112 through and center patient 112 in a gantry opening 138.Sensors 140 are positioned within gantry opening 138 to collect patientposition and contour data. Sensors 140 are connected to a sensorcontroller 142 that controls the operation of sensors 140 and providesthe acquired data to computer 126 to be processed.

As shown in FIGS. 7 and 8, an x-ray system 150 incorporating the presentinvention is shown. The x-ray system 150 includes an oil pump 152, ananode end 154, and a cathode end 156. A central enclosure 158 isprovided and positioned between the anode end 154 and the cathode end156. Housed within the central enclosure 158 is an x-ray generatingdevice or x-ray tube 160. A fluid chamber 162 is provided and housedwithin a lead lined casing 164. Fluid chamber 162 is typically filledwith coolant 166 that will be used to dissipate heat within the x-raygenerating device 160. Coolant 166 is typically a dielectric oil, butother coolants including air may be implemented. Oil pump 152 circulatesthe coolant through the x-ray system 150 to cool the x-ray generatingdevice 160 and to insulate casing 164 from high electrical charges foundwithin vacuum vessel 168. To cool the coolant to proper temperatures, aradiator 170 is provided and positioned at one side of the centralenclosure 158. Additionally, fans 172, 174 may be mounted near theradiator 170 to provide cooling air flow over the radiator 170 as thedielectric oil circulates therethrough. Electrical connections areprovided in anode receptacle 176 and cathode receptacle 178 that allowelectrons 179 to flow through the x-ray system 150.

Casing 164 is typically fanned of an aluminum-based material and linedwith lead to prevent stray x-ray emissions. A stator 180 is alsoprovided adjacent to vacuum vessel 168 and within the casing 164. Awindow 182 is provided that allows for x-ray emissions created withindie system 150 to exit the system and be projected toward an object,such as, a medical patient for diagnostic imaging. Typically, window 182is formed in casing 164. Casing 164 is designed such that most generatedx-rays 184 are blocked from emission except through window 182. X-raysystem 150 includes an attenuation filter assembly 186 designed tocontrol an attenuation profile of x-rays 184.

As stated, the present invention provides a means to determine patientparticulars such as patient size, shape, and centering from one or twoscout scans. The information is used to provide centering information tothe user, allow user selection of a ROI, automatically center thepatient elevation, correct projection area measurements for dynamic tubecurrent control, and select the correct bowtie filter configuration forthe optimum dose efficiency. The methods include automatic selection ofthe proper bowtie filter opening to control the impact of the bowtiefilter and patient mis-centering on tube current or x-ray fluxmodulation.

Referring now to FIG. 9, a flow chart setting forth the steps of animaging technique in accordance with the present invention is shown. Thetechnique is particularly tailored for dynamic bowtie and tube currentcontrol. The technique begins at 200 with the performance of at leastone scout scan and/or sensing a patient elevational profile 202 todetermine a required tube current modulation 204 in the x, y andz-directions for a desired image noise assuming a properly centeredpatient. As will be described in detail, the scout scan(s) may be alateral scout scan or an anterior-posterior (AP) or posterior-anterior(PA) scout scan. Depending on the orientation of the available scoutscan(s), a starting CT scan angle, a location in the z-direction, andpositions for the left and right filter segments of the continuouslyvariable bowtie filter, such as that described in commonly assignedpatent application U.S. Ser. No. 10/605,789, are selected 206. Thestarting bowtie (attenuation) filter positions are determined 206independently for each side, as will be described with respect to FIG.10.

Once the starting bowtie filter positions have been set 206, scanningbegins 208. Bowtie position information is collected and included foreach projection during the scan to allow the bowtie attenuation profileto be properly normalized during image reconstruction. Bowtiepositioning repeatability is preferably maintained within tenmicrometers to allow dynamic calibration and correction of the movingbowtie during patient scanning.

That is, during the scan 208 the information from the scout scan(s)and/or sensed patient elevational profile 202 is/are used to adjustoperating parameters. Specifically, a maximum edge x-ray flux is sensed210 and a closed loop feedback system is utilized to determine whethersuch is within a select range 212. If it is determined that the maximumedge x-ray flux is outside the selected range 214, the bowtie filter isadjusted to maintain the maximum edge x-ray flux 216. That is, as themaximum flux at the edge of the imaging object increases, an associatedfilter segment of the bowtie filter is moved toward isocenter.Conversely, if the flux at the edge relative to the center flux is belowthe selected range, an associated filter segment of the bowtie filter ismoved away from isocenter. However, if it is determined that the maximumedge x-ray flux is inside the selected range 218, the bowtie filter isnot adjusted and sensing of the maximum edge x-ray flux continues.

At the same time, a mean x-ray flux rate at the central portion of theimaging subject is sensed 220 and a determination of whether the meanx-ray flux rate at the central portion of the imaging subject is outsidea selected range is made at 222. If the mean x-ray flux rate at thecentral portion of the imaging subject is outside the selected range224, the tube current (mA) is adjusted to maintain the desired meanx-ray flux rate in the central projection region of the imaging subject226. On the other hand, if the mean flux rate at the central portion ofthe imaging subject is within the selected range 228, no change to tubecurrent is made and sensing 220 continues.

However, since mA modulation influences the edge flux, it iscontemplated that the control of edge flux levels may be done relativeto the average central flux level. As such, in accordance with oneembodiment of the present invention, the adjustment of bowtie filtertoward isocenter 216 and the adjustment of tube current 226 are based onan interdependent consideration of both the sensed maximum flux at theedge of the imaging object 210 and the sensed mean flux rate at thecentral portion of the imaging object 210.

Furthermore, as will be described, it is contemplated to use a prioripositioning for dynamic bowtie positioning with the feedback loop and touse filter positioning moves to prevent photon pileup only when absoluteflux limitations are at risk thereby also compensating for the fact thatmA modulation influences the edge flux. In this way, the bowtie filtercan be positioned for optimum dose efficiency based on imaging subjectsize, shape, and centering as a first priority whereby positioning forprevention of photon pileup during scanning has precedence. However, itis contemplated that for situations where the central projection regionmay have the highest x-ray flux, such as for AP projections whenscanning legs, for example, adjusting the mA to avoid photon pileup inthe center of the projections may be given priority over the tubecurrent modulation objectives.

As will be shown, reliable patient size and centering determinations canbe made from projections using two orthogonal scout scans or a singlescout scan. However, as will be shown, the present invention includessystems to compensate for the absence of a second scout scan byaccurately sensing patient elevational contours. The present inventionalso includes a method of improved calculations of subject centerwhereby the centroid (center of mass) is determined from two orthogonalscout projections or estimated from a single scout scan.

The method of FIG. 9 may be utilized to maintain the x-ray flux ratesbelow an absolute maximum limit of a detector. That is, x-ray rates forsome detector configurations may be significantly lower than otherdetectors. Hence, flux rates must be carefully managed to avoid countrate saturation (photon pileup). Since patient attenuation, projectioncentering error, and desired flux rate levels are known; x-ray fluxrates can be controlled by appropriate filter positioning and tubecurrent adjustments using expressions representing the fundamental x-rayphysics attenuation and absorption equations.

Referring now to FIG. 10, an example of a patient mis-centering to theleft is illustrated 230. That is, the patient 232 is mis-centered withrespect to the isocenter 234 of the x-ray flux passing through a bowtiefilter configuration 236. The bowtie filter configuration 236 includes aleft bowtie portion 238 and a right bowtie portion 240 that aredynamically adjustable by left control motor 242 and right control motor244. The bowtie configuration and the tube current are controlled suchthat a central patient region 246 is within a desired object flux 248according to a desired image noise. Furthermore, the left bowtie portion238 is positioned to maintain a left patient edge region flux profile250 below a max flux limit 252. Similarly, the right bowtie portion 240is positioned to maintain a right patient edge region flux profile 254below the max flux limit 252. Accordingly, a filter and patient x-rayflux profile 256 has a relative flux level 258 below the max flux limit252. As such, flux rates are maintained to remain under the x-ray ratesrequired for specific detector configurations, thereby avoiding photonpileup.

Specifically, the system may be used to overcome limitations, such asphoton pileup, which is commonly encountered with the use of photoncounting (PC) and photon energy discriminating detectors (ED) CT asopposed to traditional photon energy integrating CT detectors. Photoncounting CT systems include detector systems that are capable ofdistinguishing between photons such that a photon is differentiated fromanother photon and counted as it is received by the detector. Energydiscriminating CT systems are capable of tagging each photon count withits associated energy level. As will be described in detail below, thepresent invention provides a means to determine an imaging subject'ssize, shape, and centering and to use this information to providecentering information for automatically re-center patient elevation.Accordingly, as shown in FIG. 10, x-ray flux management may becontrolled to maintain a flux profile 250, 254 that is below a max fluxlimit 252 of a specific detector and its respective flux limits. Forexample, the flux profile 250, 254 may be specifically controlled tosatisfy the requirements of ED or PC CT detectors so as to avoid photonpileup.

Referring now to FIG. 11, FIG. 11 provides a detailed method foradjusting pre-imaging and imaging parameters. Specifically, two scoutscans are performed 300 that include an AP scout scan and a lateralscout scan. From the two scout scans a centroid projection 302 is made.Specifically, the distance of the centroid from a point of reference ismade. In a preferred embodiment, the point of reference is isocenter ofthe x-ray fan beam and the distance of the centroid from isocenter isdetermined. However, it is also contemplated that the point of referencemay be the center of the medical imaging device or the center of thebore of the medical imaging device, or any other stationary point thatis readily identifiable. Additionally, it is contemplated that the pointof reference may be a map of an ideally positioned imaging subject withsimilar physical features. In any case, the distance of the centroidfrom the point of reference is used to geometrically calculate an x andy centering error for the patient relative to a reference position 304.In accordance with a preferred embodiment of the present invention, thereference position is at a center of the scanning bay located in they-direction. However, it is also contemplated that the referenceposition may be arbitrarily selected as long as the reference positionis fixed with respect to patient position within the CT bore. Havingcalculated the y-axis patient centering error over the extent of theprescribed CT scan, the system determines the mean center with respectto the reference position, to provide the optimum fixed table height forthe duration of the CT scan. The x and y mis-centering is then comparedto a threshold 305. Accordingly, a direct determination of the center ofthe imaging subject is made. That is, by utilizing centroid calculationsthe center of maximum attenuation that should be positioned in themaximum x-ray field is determined rather than the physical centerrelative to the edges of the object.

If the mis-centering is less than the threshold 306, indicating that thecurrent position of the patient is within the imaging tolerance of thesystem, no adjustment is necessary and the system is ready for scanning307. However, if the mis-centering is greater than the threshold 308,the operator is notified of the centering error 309 and presented withan auto-correction prompt whereby the operator is prompted to accept orreject 310 the table elevation change. However, it is contemplated thatoperator approval may be bypassed whereby auto-correction is completedwithout operator approval 310. As such, a fully automated correctionsystem may be implemented. As will be described with respect to FIGS. 11and 12, should the operator reject the auto-correction 311, the operatormay use a graphical indication or other means to enter a user-selectedcentering correction 312 according to which scanning is performed 313.

Should the operator accept the auto-correction 314, the patientelevation is automatically corrected 316. Additionally, the x and ycentering errors are used to correct the projection area (PA) 318. ThePA is the sum of the attenuation values of the x-rays that intercept thepatient. Therefore, PA is dependent on the distance of the patient fromthe fan beam x-ray source. By utilizing the accuracy of the centroidcalculated x and y centering errors, a corrected PA is directlycalculated using geometric equations 318. The PA from both the AP andlateral scouts can be corrected using the centering error determinedfrom the orthogonal scout scan and the average AP scout scan. That is,lateral PA can be used to improve the accuracy of a tube currentmodulation noise prediction algorithm 322. Additionally, the oval ratio(OR) is directly computed 320 using the projection measure (PM) ratiofrom the two orthogonal scouts to further improve the accuracy of thetube current modulation noise prediction 322. That is, the tube currentis then boosted to compensate for the centroid calculated mis-centering324.

Once adjustments according to the centroid calculated mis-centering arecomplete 316–324, the proper bowtie filter configuration is selected326. Specifically, for a given bowtie filter shape and a given patientsize and shape there exists an optimum opening, measured in flat width(FW), that provides the best image quality at the lowest dose. Theoptimum value is the value of FW that maximizes a quality factor Q ascalculated as follows:

${Q = \frac{{KC}\left( {a,b,{FW}} \right)}{{N\left( {a,b,{FW}} \right)}\sqrt{D\left( {a,b,{FW}} \right)}}};$where:

N is the overall noise in the image or scan data (standard deviation);

D is the dose to the object;

C is the contrast between two materials such as iodine and water(dependent on the spectral characteristics of the system);

K interpolates linearly between

${Q = {{\frac{1}{\sqrt{N^{2}D}}\mspace{25mu}{and}\mspace{20mu} Q} = \frac{C}{\sqrt{N^{2}D}}}};$

a and b are the axes parameters for an ellipse; and

FW is one half of the flat width (i.e. ½ the length of the uniform lowattenuation region of the bowtie filter in mm).

In accordance wit an alternative embodiment, quality factor may bedetermined using a single diameter parameter d, where d is the averageof a and b. In either case, once the proper bowtie filter configurationis selected 326, the system is ready for scanning 328. As such, thepatient table is raised or lowered dynamically during the execution of ahelical CT scan to accommodate the changing optimum elevations dependingon patient anatomy and centering/mis-centering. Elevation data isincluded in the scan data header to properly position the views duringimage reconstruction. If a continuous bowtie is present, the bowtie ispositioned dynamically to follow the sineogram of the patient. That is,an attenuation pattern may be utilized that maps a dynamic configurationof the attenuation of the bowtie so as to achieve desired attenuationover time, i.e. during data acquisition.

Referring now to FIG. 12, the optimum bowtie filter opening can bedetermined experimentally by constructing various phantom sizes andshapes and then scanning the phantoms with various bowtie filters havingdifferent FW values, reconstructing images, measuring the noise, dose,and contrast for each case, and fitting a curve to the Q values vs. FWas shown in FIG. 13. The optimum FW value for a given patient size canthen be determined by reviewing FW value against the Q value.Specifically, the FW value where Q is at a maximum is the optimum FWvalue, as illustrated in FIG. 13. The Q values can also be determined bycomputer modeling using fundamental x-ray physics attenuation andabsorption equations to estimate the noise, contrast, and dose in theimage for each case. The contrast weighting value K can be chosenbetween 0 and 1. In the given example, the value of K is zero in orderto exclude any benefits of improved object contrast.

From experimental data or simulations the set of optimum bowtie openingvalues can be determined versus patient or object size as shown in FIG.12. The relationship is approximately linear and can be represented bythe equation FW=0.45 (d-10) for the K=0 assumption where d is thepatient diameter in centimeters. Patients with diameters less than 10 cmwould use a bowtie opening FW value of zero. As such, optimum bowtieopening FW can be accurately selected given the patient diameter d. Thepatient diameter can be determined from the PM (amplitude of projection)and a patient density assumption μ. The average PM can be obtained fromthe orthogonal scout scan pair since d=avg(PM(μ). For the human body,the density assumption μ can be assumed to be 0.2, which is theattenuation coefficient of water, except for the chest and head. For thechest and head, μcan be approximated as 0.14 and 0.24, respectively, dueto the density decrease of the lungs and the density increase of theskull.

For CT systems with a continuously variable addressable bowtie, the FWvalue can be determined directly by the equation, d=avg(PM/μ). On CTsystems without an addressable continuous bowtie, the equation can beused to select the nearest optimum bowtie from the selection ofavailable discrete bowtie filters. For example a set of discrete bowtiefilters that covers the patient range from infants to large obese adultswould typically include bowtie filters with openings having FW values of1, 5, 9, and a flat filter. From the graph on FIG. 12, the followinglookup table can be constructed to automatically select the most optimumdiscrete bowtie for the patient as follows:

DIAMETER: <15 cm 15 to 25 cm >25 to 35 cm >35 cm BOWTIE FILTER: FW 1 FW5 FW 9 Flat

The optimum filter opening, however, is dependent on how well thepatient is centered in addition to the patient's diameter. The effect ofpatient mis-centering is comparable to a patient radius increase for theprojections perpendicular to the mis-centering axis. Hence the properfilter selection is a function of the patient diameter plus themis-centering and can be determined using the equation, FW=0.45(d-10+2ew), where e is the patient mis-centering error in centimetersand w is a weighting factor or function. The weighting factor istypically 1.0 but could be less than 1.0 to constrain the dose increasethat would otherwise result when the bowtie is opened to fully accountfor the worst case effect of mis-centering. The value of w could also bea function of the object size, shape, and mis-centering to more closelymatch the behavior of image noise with mis-centering of various sizeobjects.

A discrete bowtie selection can also be obtained by adding the centeringerror factor (2ew) to the phantom diameter for the lookup table index.For example, from the table, a 24 cm patient with a 3 cm error would beconsidered a 30 cm diameter and hence, filter FW 9 should be selectedinstead of FW 5 for the centered case. Furthermore, in the event thattube current modulation is used and the patient is mis-centered in asmaller than optimum bowtie, the mA can be boosted to avoid anunacceptable noise increase in the image.

Referring now to FIG. 14, an example of the user interface through whichmanual entry of a user-selected centering correction 312, FIG. 11, maybe entered is shown. In the given example, the user is performing aspine study. In this case, a spine study is optimally centered on thespine 410 instead of the overall attenuation centroid for the patient412. However, the automatically calculated adjustments will be basedupon the mean center of the patient over the scan length and yield theoverall attenuation centroid of the patient as the center point 412.Accordingly, the automatically calculated adjustments based on thecentroid calculations to compensate for mis-centering are not optimalfor the spine study and the operator will choose to manually enter auser-selected centering correction such that recentering is along themean center of the spine over the scan length 414.

Referring to FIG. 15, another view of the example of the user interfacethrough which manual entry of a user-selected centering correction maybe entered on a pair of scout scans is shown. Through the interface, theuser marks the location of the spine or other area of interest on scoutscans using cursor markers 416. Via the user-defined cursor markers 416,a diameter of interest 418 is defined that includes a center of interest420 independent of the centroid calculated isocenter 422.

Accordingly, the patient table may be raised or lowered dynamicallyduring the execution of a CT scan to accommodate the changing optimumelevations depending on patient anatomy to track the user-defined cursormarkers. Elevation data is included in the scan data header to properlyposition the views during image reconstruction. If a continuous bowtiefilter is present, the bowtie filter may be controlled dynamically tofollow the sineogram of the patient. That is, if the location of a ROIis designated 418 via markers 416, the bowtie filter is dynamicallypositioned to follow the sineogram of the ROI. This positioning obtainsimproved image quality for the ROI and reduces dose elsewhere.

FIG. 16 illustrates an implementation of the method illustrated in FIG.11 when only a lateral patient scout scan 510 is available. From thelateral scout scan 510, a centroid projection is made at 512 and ymis-centering is determined relative to a reference position 514according to the methods previously described. However, since no APscout scan data is available, x mis-centering is assumed to be zero. Theassumption that x mis-centering is 0 provides a reasonable estimation aslong as the operator utilizes the edges of the patient table as a guidewhen positioning the patient in x. Then, having determined the y axispatient centering error over the extent of the prescribed CT scan, thesystem determines the mean center to provide the optimum fixed tableheight for the duration of the CT scan. The y mis-centering is thencompared to a threshold 516.

If the mis-centering is less than the threshold 518, indicating that thecurrent position of the patient is within the imaging tolerance of thesystem, no adjustment is necessary and the system is ready for scanning520. However, if the mis-centering is greater than the threshold 522,the operator is notified of the centering error 524 and presented withan auto-correction prompt whereby the operator is prompted to accept orreject 526 the table elevation change. However, it is contemplated thatoperator approval may be bypassed whereby auto-correction is completedwithout operator approval 526. As was described with respect to FIGS. 11and 12, should the operator reject the auto-correction 528, the operatormay use a graphical indication or other means to enter a user-selectedcentering correction 530 according to which scanning is performed 532.

Should the operator accept the auto-correction 534, the patientelevation is automatically corrected 536. The PA, PM, and OR arecalculated 538–540 from the single scout using known methods utilizing ymis-centering calculations. However, since only one PM is available fromthe single scout scan, the diameter for bowtie selection is determinedby the equation, d=(PM/μ)(OR+1)/2 because the OR, by definition, is theratio of the axis parameters of the elliptical patient model. As such,mA modulation is calculated 542, the mA boost factor is implemented 544,and the appropriate bowtie is selected 546 based on the y-axis centeringinformation using the methods previously described herein. Accordingly,scanning is performed at 548.

FIG. 17 illustrates an implementation of the method illustrated in FIG.11 when only an AP patient scout is available. Fundamentally, the methodshown in FIG. 17 is substantially similar to that of FIG. 16; however,the y-axis centering error can not be directly determined since it is inthe same orientation as the scout projections. Nevertheless, an estimateof the y-axis error relative to a reference position can be made ifelevation information relative to the surface of the patient isavailable.

Once the AP scout scan is complete 610, the system determines whetherthe surface elevation of the patient is known 612. If the surfaceelevation of the patient is unknown 614, the operator is prompted tomanually select a bowtie filter configuration and calculate tube currentper traditional manual methods 616 and a scan is performed 617. However,if the surface elevation of the patient is known or derived 618, as willbe described with respect to FIGS. 18–20, an estimation of ymis-centering is performed 620.

The estimation of y mis-centering is then compared to a threshold 622.If the mis-centering is less than the threshold 624, indicating that thecurrent position of the patient is within the imaging tolerance of thesystem, no adjustment is necessary and the system is ready for scanning626. However, if the mis-centering is greater than the threshold 628,the operator is notified of the centering error 630 and presented withan auto-correction prompt whereby the operator is prompted to accept orreject 632 the table elevation change. However, it is contemplated thatoperator approval may be bypassed whereby auto-correction is completedwithout prior operator approval 632. As was described with respect toFIGS. 11 and 12, should the operator reject the auto-correction 634, theoperator uses a graphical indication or other means to enter auser-selected centering correction 636 according to which scanning isperformed 638.

Should the operator accept the auto-correction 640, the patientelevation is automatically corrected 642. The PA is then corrected 644for the estimated y-axis centering error. This is done by directgeometric calculations or as a fitted function of elevation, PA, and OR,as will be described with respect to FIG. 18–20. As such, mA modulationis determined 646, the mA boost factor is implemented 648, and theappropriate bowtie is selected 650 based on the y-axis centeringinformation using the methods previously described herein. Accordingly,the system is ready for scanning 652.

However, it is also contemplated that estimations for PA, PM andmis-centering may be generated from the surface contour of the patient.As such, it is possible to determine mA modulation, boost mA tocompensate for patient mis-centering, and select a desired bowtieconfiguration without the benefit of scout scans. That is, for theselection of the bowtie filter configuration, it is assumed that thepatient is centered and the bowtie configuration is selected based onpatient size estimated from the PM and density assumption of μ aspreviously described herein.

Referring to FIGS. 18, 19, and 20, surface elevation information aboutthe patient can be obtained by various methods. If the patient 706 isresting directly on the patient table, as in FIG. 18, the tableelevation can be used to determine y-axis centering error. Specifically,with respect to FIG. 18, the table height 708 is known and, as such, theupper horizontal axis 710 of the patient 706 is known or reasonablyestimated. Therefore, once the vertical axis 712 is determined, asdescribed above, the upper center 714 of the patient 706 can bedetermined from the intersection of the upper horizontal axis 710 of thepatient 706 and the vertical axis 712. Accordingly, the center 716 ofthe patient 706 is disposed halfway between the upper center 714 and thetable height 708. Given the determination of these values, tableelevation relative to isocenter (E) can be calculated by solving for theequation E=R+H−C, wherein H is the height of the table 714, R is thedifference between the center 716 of the patient 706 and the tableheight 708, and C is the height of the upper center 714 of the patient706. Specifically, mis-centering is determined by measuring the offsetof the contour projections from isocenter.

However, in cases where the patient 706 is propped up, as in FIG. 19,with pillows or other positioning devices 718, the centering can bedetermined from a laser or sonic displacement measuring devicepositioned on the gantry or otherwise disposed on the scanner to locatethe top surface of the patient 706. As such, a vector of positioninformation is collected and associated with each scout projection toallow the centering error to be calculated as a function of thez-direction. Specifically, since the center 716 of the patient 706cannot readily be readily discerned because it is not disposed halfwaybetween the upper center 714 and the table height 708 due to the offsetcreated by the positioning device 718, as shown in FIG. 18, a laser orsonic displacement sensor 720 may be utilized to determine a distance Lto the upper horizontal axis 710 of the patient 706. As such, E can becalculated in this case according to: E=C−R−L.

However, referring to FIG. 20, it is also contemplated that a pluralityof lasers and/or sonic displacement 720 sensors may be utilized tomeasure the distance from an array of points to obtain the specificcontour of the patient 706. As such, an improved accuracy determinationof overall patient contour is achieved.

In any case, the PA can be determined from the external patient contourand the μ for the associated anatomy as described previously herein. TheOR is determined directly from the distance measurements or from the PMwhich can be determined from the μand patient surface distances.Mis-centering is determined by measuring offset of the contourprojections from isocenter.

Once the contour of the patient is known, it is possible to calculatethe projection error ratio and fit it to a cubic or other function ofelevation, PA, and OR to determine equation coefficients in order tocalculate the PA corrected for y-axis centering error according to thefollowing:

PA = P/C1 + (C2 * E) + (C3 * P) + (C4 * O) + (C5 * E * P) + (C6 * E * O) + (C7 * P * O) + (C8 * E²) + (C9 * P²) + (C10 * O²)+)C11 * E * P * O) + (C12 * E² * P) + (C13 * E²O) + (C14 * P² * E) + (C15 * P² * O) + (C16 * O² * P) + (C17 * O² * P) + (C18 * E³);wherein:

Eq coeff Variable C1 constant C2 elevation C3 PA C4 OVR C5 elevation *PA C6 elevation * OVR C7 PA * OVR C8 elevation² C7 PA² C10 OVR² C11elevation * PA * OVR C12 elevation² * PA C13 elevation² * OVR C14 PA² *elevation C15 PA² * OVR C16 OVR² * elevation C17 OVR² * PA C18 OVR³ andE is the table elevation relative to isocenter; P is the measuredprojection area; PA is the projection area corrected for table/patientelevation; and O is the oval ratio.

It is contemplated that the above-described invention be utilized with“third generation” CT systems as well as a wide variety of other CT-typesystems. That is, it is contemplated that the present invention may beutilized with energy integrating, PC, and ED CT detector systems.Furthermore, it is contemplate that the above-described invention may beutilized with non-traditional and non-medical CT applications. Forexample, it is contemplated that the above-described invention may beutilized with a non-invasive package/baggage inspection system, such asthe system shown in FIG. 21.

Referring now to FIG. 21, package/baggage inspection system 800 includesa rotatable gantry 810 having an opening 812 therein through whichpackages or pieces of baggage may pass. The rotatable gantry 810 housesa high frequency electromagnetic energy source 814 aligned with anattenuation filter 815 as well as a detector assembly 816. A conveyorsystem 818 is also provided and includes a conveyor belt 820 supportedby structure 822 to automatically and continuously pass packages orbaggage pieces 824 through opening 812 to be scanned. Objects 824 arefed through opening 812 by conveyor belt 820, imaging data is thenacquired, and the conveyor belt 820 removes the packages 824 fromopening 812 in a controlled and continuous manner. As a result, postalinspectors, baggage handlers, and other security personnel maynon-invasively inspect the contents of packages 824 for explosives,knives, guns, contraband, and the like.

Therefore, in accordance with one embodiment of the current invention, amethod of diagnostic imaging is disclosed that includes determining aposition of a subject in a scanning bay relative to a referenceposition, automatically adjusting an attenuation characteristic of anattenuation filter based on the determined position of the subject andimaging the subject.

In accordance with another embodiment of the invention, a computerreadable storage medium is disclosed that has stored thereon a computerprogram representing a set of instructions. When the instructions areexecuted by at least one processor, the at least one processor is causedto receive feedback regarding mis-centering of a subject to be scanned,determine a value of mis-centering of the subject to be scanned, andadjust at least one of an attenuation filter configuration and a subjectposition based on the value of mis-centering. The processor is thencaused to acquire radiographic diagnostic data from the subject.

In accordance with still another embodiment of the invention, atomographic system is disclosed. The tomographic system includes arotatable gantry having a bore centrally disposed therein, a tablemovable within the bore and configured to position a subject fortomographic data acquisition within the bore, and a high frequencyelectromagnetic energy projection source positioned within the rotatablegantry and configured to project high frequency electromagnetic energytoward the subject. A detector array is disposed within the rotatablegantry and configured to detect high frequency electromagnetic energyprojected by the projection source and impinged by the subject and anattenuation filter positioned between the high frequency electromagneticenergy projection source and the subject. A computer is programmed toadjust at least one of an attenuation characteristic of the attenuationfilter and a table position based on a specific position of the subjectin the bore.

In accordance with yet another embodiment of the invention, a method ofcentering a subject in a medical imaging device is disclosed thatincludes positioning a subject in a scanning bay, comparing a center ofmass of the subject to a reference point, and repositioning the subjectin the scanning bay to reduce a difference in position between thecenter of mass of the subject and the reference point.

In accordance with another embodiment of the invention, a computerreadable storage medium having stored thereon a computer programrepresenting a set of instructions is disclosed. The instructions, whenexecuted by at least one processor, causes the at least one processor todetermine a centroid of a subject, determine a value of mis-centering ofthe centroid of the subject within a medical imaging device, and adjusta position of the subject within the imaging device to compensate forthe value of mis-centering.

In accordance with yet another embodiment of the invention, a method ofmedical imaging is disclosed that includes positioning a subject in amedical imaging device, determining a value of mis-elevation of thesubject, and adjusting an elevation of the subject device to reduce thevalue of mis-elevation.

In accordance with still another embodiment of the invention, atomographic system is disclosed that includes a rotatable gantry havinga bore centrally disposed therein, a table movable within the bore andconfigured to position a subject for tomographic data acquisition withinthe bore, and a high frequency electromagnetic energy projection sourcepositioned within the rotatable gantry and configured to project highfrequency electromagnetic energy toward the subject. The tomographicsystem also includes a detector array disposed within the rotatablegantry and configured to detect high frequency electromagnetic energyprojected by the projection source and impinged by the subject andcomputer. The computer is programmed to determine a centroid of thesubject and adjust an elevation of the subject to align the centroidwith a reference position.

In accordance with one embodiment of the invention, a method of imagingis disclosed that includes positioning a subject in an imaging device,performing at least one scout scan, and marking a user-definedregion-of-interest (ROI). An attenuation characteristic of anattenuation filter is then automatically adjusted based on theuser-defined ROI.

In accordance with another embodiment of the invention, a tomographicsystem is disclosed that includes a rotatable gantry having a borecentrally disposed therein, a table movable within the bore andconfigured to position a subject for tomographic data acquisition, and ahigh frequency electromagnetic energy projection source positionedwithin the rotatable gantry and configured to project high frequencyelectromagnetic energy toward the subject. A detector array is disposedwithin the rotatable gantry and is configured to detect high frequencyelectromagnetic energy projected by the projection source and impingedby the subject. An attenuation filter is positioned between the highfrequency electromagnetic energy projection source and the subject. Acomputer is included that is programmed to display a user interfaceincluding an illustration of a position of the subject and allowselection of a ROI and determine an attenuation profile of theattenuation filter based on the user-selected ROI.

In accordance with another embodiment of the invention, a computerreadable storage medium having stored thereon a computer programrepresenting a set of instructions is disclosed. The instructions, whenexecuted by at least one processor, cause the at least one processor toperform at least one scout scan, display an interface including areconstructed image from the at least one scout scan and receiveuser-selection identifying a ROI. The instructions then cause the atleast one processor to adjust at least one of an attenuation filterconfiguration and a subject position based on the ROI.

In accordance with yet another embodiment of the invention, atomographic system is disclosed that includes a rotatable gantry havinga bore centrally disposed therein, a table movable within the bore andconfigured to position a subject for tomographic data acquisition withinthe bore, and a high frequency electromagnetic energy projection sourcepositioned within the rotatable gantry and configured to project highfrequency electromagnetic energy toward the subject. A detector array isdisposed within the rotatable gantry and configured to detect highfrequency electromagnetic energy projected by the projection source andimpinged by the subject and at least one sensor is included to providesubject position feedback.

In accordance with another embodiment of the invention, a computerreadable storage medium is disclosed having stored thereon a computerprogram representing a set of instructions. When the instructions areexecuted by at least one processor, the at least one processor is causedto receive feedback regarding a subject position from at least onesensor of an imaging device and determine a centering error from thefeedback.

In accordance with one more embodiment of the invention, a method ofimaging is disclosed that includes positioning a subject in an imagingdevice, collecting positioning information of the subject from at leastone sensor disposed in proximity of the imaging device, and determininga relative position of the subject within the imaging device from atleast the position information.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A method of diagnostic imaging comprising the steps of: comparing aposition of a subject in a scanning bay relative to a referenceposition; determining a region of maximum attenuation of the subjectfrom the comparison; automatically adjusting an attenuationcharacteristic of an attenuation filter based on the determined regionof maximum attenuation of the subject; and imaging the subject.
 2. Themethod of claim 1 further comprising determining a size and an elevationof the subject within the scanning bay.
 3. The method of claim 2 furthercomprising adjusting the attenuation characteristic of the attenuationfilter according to the size and the elevation of the subject.
 4. Themethod of claim 1 further comprising automatically adjusting theelevation of the subject within the scanning bay to optimize radiationexposure to the subject.
 5. The method of claim 1 further comprisingacquiring data from at least one scout scan to determine the position ofthe subject in the scanning bay.
 6. The method of claim 5 furthercomprising determining at least one of a size, a shape, and a centeringof the subject from the at least one scout scan.
 7. The method of claim5 wherein the step of acquiring data from at least one scout scanincludes acquiring a flux trend of the scout scan and wherein the stepof adjusting an attenuation characteristic of an attenuation filterincludes adjusting a filter position according to the flux trend.
 8. Themethod of claim 7 wherein the step of adjusting an attenuationcharacteristic of an attenuation filter includes at least one of:adjusting a position of the attenuation filter to avoid flux ratesbeyond a threshold rate; and adjusting a position of the attenuationfilter according to a flux rate of a central region of the subject. 9.The method of claim 1 wherein the step of adjusting an attenuationcharacteristic of an attenuation filter includes configuring an imagingfilter to provide an optimal dose profile of high frequencyelectromagnetic energy to the subject.
 10. The method of claim 1 furthercomprising modulating a high frequency electromagnetic energy projectionsource at least according to the position of the subject in the scanningbay.
 11. The method of claim 1 further comprising performing at leastone orthogonal scout and performing centroid calculations to determine acenter of the subject.
 12. The method of claim 1 further comprisingdetermining a diameter of the subject and an optimum bowtie filteropening for the diameter of the subject.
 13. The method of claim 1further comprising determining a contour of the subject and the positionof the subject in the scanning bay according to feedback from at leastone of a laser sensor and a sonic sensor.
 14. The method of claim 13further comprising determining an area of the subject from the contourof the subject.
 15. The method of claim 1 further comprising determininga position of the subject in three dimensions.
 16. A computer readablestorage medium having stored thereon a computer program representing aset of instructions which, when executed by at least one processor,causes the at least one processor to: receive feedback regarding aposition of maximum attenuation of a subject to be scanned; determine avalue of mis-centering of the subject to be scanned from the position ofmaximum attenuation relative to an isocenter of an x-ray beam; adjust atleast one of an attenuation filter configuration and a subject positionbased on the value of mis-centering; and acquire radiographic diagnosticdata from the subject.
 17. The computer readable storage medium of claim16 wherein the at least one processor is further caused to repeatedlyreceive position information about the attenuation filter during theacquisition of radiographic diagnostic data from the subject.
 18. Thecomputer readable storage medium of claim 16 wherein the at least oneprocessor is further caused to determine a desired tube currentmodulation in a first, a second, and a third direction with respect to adesired image noise and dynamically adjust a tube current based on thedesired tube current modulation.
 19. The computer readable storagemedium of claim 16 wherein the at least one processor is further causedto determine a center of mass of the subject and determine a distance ofthe center of mass from isocenter.
 20. The computer readable storagemedium of claim 19 wherein the at least one processor is further causedto determine a centering error from the distance of the center of massof the subject from isocenter.
 21. The computer readable storage mediumof claim 20 wherein the at least one processor is further caused toadjust a projection area according to the determined centering error.22. The computer readable storage medium of claim 16 wherein the atleast one processor is caused to adjust the position of the subject byadjusting an elevation of the subject.
 23. The computer readable storagemedium of claim 16 wherein the at least one processor is further causedto determine an optimum opening of the attenuation filter to optimizethe acquisition of radiographic diagnostic data from the subject whilereducing dosage of electromagnetic energy projected toward the subject.24. A tomographic system comprising: a rotatable gantry having a borecentrally disposed therein; a table movable within the bore andconfigured to position a subject for tomographic data acquisition withinthe bore; a high frequency electromagnetic energy projection sourcepositioned within the rotatable gantry and configured to project highfrequency electromagnetic energy toward the subject; a detector arraydisposed within the rotatable gantry and configured to detect highfrequency electromagnetic energy projected by the projection source andimpinged by the subject; an attenuation filter positioned between thehigh frequency electromagnetic energy projection source and the subject;and a computer programmed to: determine a region of maximum attenuationof the subject; and adjust at least one of an attenuation characteristicof the attenuation filter and a table position such that a region ofminimum attenuation of the attenuation filter is aligned with the regionof maximum attenuation of the subject.
 25. The tomographic system ofclaim 24 wherein the computer is further programmed to determine a meanhigh frequency electromagnetic energy at a central portion of thesubject with respect to a desired image noise, and dynamically adjust atube current to maintain the desired mean high frequency electromagneticenergy at at least one of the central portion of the subject and an edgeportion of the subject.
 26. The tomographic system of claim 24 whereinthe computer is further programmed to adjust the attenuationcharacteristic to reduce noise.
 27. The tomographic system of claim 24wherein the attenuation filter is a bowtie filter having multiplefiltering elements dynamically positioned within an x-ray path.
 28. Thetomographic system of claim 24 wherein the computer is furtherprogrammed to perform an imaging scan.
 29. The tomographic system ofclaim 28 wherein the computer is further programmed to sense a maximumedge x-ray flux and determine whether the maximum edge x-ray flux iswithin a selected range.
 30. The tomographic system of claim 29 whereinthe computer is further programmed to adjust a configuration of theattenuation filter to maintain the maximum edge x-ray flux.